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Fig. 11-5 C02 content of air bubbles trapped in glacial ice from Greenland and Antarctica, showing a pre-industrial concentration of ca. 280 ppmv.

Fig. 11-6 Concentrations of gases in the smoke from an experimental fire of Trachypogon grass from Venezuela as a function of time and the stack gas temperature. The dotted line separates the flaming phase from the smoldering phase. Concentrations are in percent by volume for C02, in volume mixing ratios (ppm) for the other species (1% = 10 000 ppm). (Used with permission from Crutzen and Andreae (1990). Science 250: 1669-1678, AAAS.)

the increase rate of atmospheric methane has decreased substantially probably to an extent due to infrastructure development in Russia combined with a general decrease in the gas extraction in FSU. The main sink for methane in the atmosphere is oxidation by the hydroxyl radical ( OH).

Oxidation of methane is one of the sources of atmospheric CO. Another internal source of importance is the oxidation of terpenes and isoprenes emitted by forests (Crutzen, 1983). Important are also biomass burning activities (Crutzen and Andreae, 1990; Hao et ai, 1996; Levine, 1994). The carbon monoxide concentration in the atmosphere ranges from 0.05 to 0.20 ppmv in the remote troposphere (with considerable differences between the northern and southern hemispheres) which means that about 0.2 Pg of carbon is present as CO in the atmosphere.

Apart from C02/ CH4/ and CO there are many gases containing carbon present in the atmosphere, terpenes, isoprenes, various compounds of petrochemical origin and others. We will not discuss them further, although some, like dimethylsulfide (DMS, (CH3)2S), are of great importance in the biogeochemical cycles of other elements. The total amount of atmospheric carbon in forms other than the three discussed is estimated at 0.05 Pg C (Freyer, 1979).

11.3.2 The Hydrosphere

Oceanic carbon is mainly present in four forms: dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particulate organic carbon (POC), and the marine biota itself. The marine biota, although it is a small carbon pool with a standing crop of about 3 Pg C (De Vooys, 1979) has a profound influence on the distribution of many elements in the sea (Broecker and Peng, 1982). Primary production in the photic zone is the major input of organic carbon in the oceans (Mopper and Degens, 1979). Labile (reactive) organic compounds are efficiently reoxidized in the mixed layer, whereas less than 10% of the primary production is distributed into the reservoirs of POC and DOC. Williams (1975) has used 14C techniques to determine the average age of deep water DOC to be 3400 years. The DOC is thus clearly older than the turnover time of water in the deep oceans (100-1000 years) indicating the persistent nature of the dissolved organic compounds in the seas.

A detailed characterization of DOC is difficult to make; a large number of compounds have been detected but only a small portion of the total DOC has been identified. Identified species include amino acids, fatty acids, carbohydrates, phenols and sterols. The amount of carbon in the oceans as DOC is estimated to 1000 Pg and the amount present as POC is about 30 Pg (Mopper and Degens, 1979).

DIC concentrations have been studied extensively since the appearance of a precise analytical technique (Dyrssen and Sillen, 1967; Edmond, 1970). The aquatic chemistry of C02 has been treated extensively; reviews can be found in Skirrow (1975), Takahashi et al. (1980), and Stumm and Morgan (1981). When C02 dissolves in water it may hydrate to form H2C03(aq) which in turn dissociates to HC03 and C03~. The conjugate pairs responsible for most of the pH buffer capacity in seawater are HC0;T/C03~ and B(OH)3/B(OH)4~ (with some minor contributions from silicate and phosphate). Although the predominance of HC03 at the oceanic pH of 8.2 actually places the carbonate system close to a pH buffer minimum, its importance is maintained by the high DIC concentration (% 2 mM). Ocean water in contact with the atmosphere will, if the air-sea gas exchange rate is short compared to the mixing time with deeper waters, reach equilibrium according to Henry's Law.

Two further reactions to be considered are the ionization of water and the borate equilibrium:

H20 + B(OH)3(aq) <-> B(OH)4"(aq) + H+(aq); [H+][B(OH)4]

In order to be able to solve for hydrogen ion concentration we define total borate (SB) and total carbon (LC = DIC) as

ZB = [B(OH)3] + [B(OH)4-] SC = [H2CO;j + [HC03- ] + [CQ§ - ]

[H2C03] represents the sum of C02(aq) and H2C03. Alkalinity, a capacity factor, representing the acid neutralizing capacity of the aqueous solution, is given by the following equation (ignoring influences from some minor components like phosphate and silicate) (see also Chapter 5):

Given any two of the four quantities EC, Alk, pH, Pcov the other two can always be calculated provided appropriate equilibrium constants are available (the equilibrium constants depend on temperature, salinity and pressure). Hydrogen ion concentration, for example, be calculated from Alk and EC with the equation

Alk^ Kw [Hj i[H+]+Kb Xi[H+] +2K& 'H+\2 + [U+}Kx + KxK2

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